The present invention generally relates to a device and a method for switching optical signals and particularly relates to an optical switch for selectively establishing pathways for light beams which enter and leave the optical switch through conduits which are distributed over two parallel planes thus allowing the selective direction of a light beam in three dimensions from an incoming to an outgoing optical fibre.
According to Jerry D. Gibson, THE COMMUNICATIONS HANDBOOK, CRC PRESS, Boca Raton 1997, chapter 37, pages 513-528 the migration of photonics into switching is occurring in a variety of ways. This is happening because of the large variety of switching tasks to be performed in a modern communications network. The different switching tasks range from ensuring that major trunks have the ability to be switched from one route to another when a catastrophic accident destroys a route, to the real-time switching on a per call basis. Two generic switching applications that are in use in present networks are protection switching and call-by-call switching. Both of these applications will preferably be realised in the future with optical switching techniques.
Most of the switches currently in use require conversion of the optical signals received through optical fibers into electrical signals which are routed for example in an electronic switching fabric from an incoming line to an output line which is selected according to the corresponding signalling information. Electronic switching fabrics are described in the aforementioned Gibson reference, chapter 32, pages 425-432.
A fiber optic communications network typically operates at data rates from a few megabit per second to beyond 2.5 Gb/s. At the 2.5 Gb/s rate, several thousand digitised voice channels, each operating at 64 kb/s, can be transmitted along a single fiber using time-division multiplexing. Electronic switching fabrics which can handle the traffic in fiber telephone networks require therefore transfer capacities which are costly to realise. Electronic switching fabrics in fiber optic communications networks will therefore be replaced in the near future by optical switches acting as cross-connects which allow the bit transparent data transfer between data systems such as public telephone exchanges.
An optical switch for the transparent transfer of data is described for example in PCT Patent Application publication number WO 00/79311. This optical switch comprises, mounted on a base, at least one controllable actuator used for positioning of an optical element designed to guide an optical signal on a selected free space switching path, within an optical path plane arranged parallel to said base, from an input optical fiber to an output optical fiber. Switching therefore takes place in two dimensions over the optical path plane along which on two sides opposite to each other optical fibers are arranged in rows. The size of the optical path plane and therefore of the optical switch rapidly increases with the number of optical fibers installed. Further the lengths of the switching paths also increase so that the optical elements require a more precise adjustment. The mounting and precise alignment of each of the optical elements, mounted perpendicular to respective upright on the base and parallel to opposing optical elements by means of hinges and a carrier panel, results in corresponding production costs. Imprecise alignment may however cause transmission losses. It would therefore be desirable to improve the described optical switch. It would be desirable in particular to provide an optical switch of smaller size while allowing the connection of an increased number of optical fibres for switching purposes. It would further be desirable to design an optical switch which can be manufactured at reduced cost. Further, it would be desirable to design an optical switch which easily allows correct geometrical positioning of the optical elements and therefore avoids adjustment problems. It would also be desirable to provide an optical switch comprising simple means for precisely aligning selected optical paths between optical fibers. It is further desirable to provide a method for establishing, optimising and correctly maintaining established optical paths between optical fibers.
In accordance with the present invention, there is now provided a device for directing an optical signal from a first optical fiber along one of a plurality of selectable switching paths each terminating in a corresponding one of a plurality of second optical fibers via an optical element, the optical element being moveable by a controllable actuator (60) from a first to a second position to change the switching path of incident optical signal, characterised in that the optical element is slideably mounted in parallel to a first mounting plate comprising a conduit through which the optical signals from the first optical fiber can be directed by the optical element along the selected one of the switching paths to one of a plurality of conduits in a second mounting plate parallel to the first mounting plate, and further to the corresponding one of second optical fibers.
In a preferred embodiment of the present invention, the first mounting plate comprises a plurality of conduits each for communicating optical signals with a corresponding one of a plurality of first optical fibers and each corresponding to a different one of a plurality of optical elements slideably mounted in parallel to the first mounting plate for selectively directing optical signals passing through the corresponding conduit.
Optical elements may be mounted in parallel to the second mounting plate for adapting an optical signal arriving over the switching path to the second optical fibers. The optical elements mounted in parallel to the second mounting plate may be slideably moveable relative to the conduits in the second mounting plate for directing optical signals arriving from the second optical fibers to the optical elements of the first mounting plate and further to the first optical fibers.
Each of the slideable optical elements is preferably coupled to an actuator subsystem for sliding the optical elements between different positions within a plane. The actuator subsystem preferably moves the slideable optical elements based on forces generated by any one or more of piezoelectric, electrostatic, acoustic and thermal effects.
The actuator subsystem may hold the slideable optical elements in position by friction. Each slideable optical element preferably comprises a flat part which is supported by the actuator subsystem and at least one of a reflective part and a refractive part. The reflective part may comprise a mirror and the refractive part may comprise a lens.
In a particularly preferred embodiment of the present invention, the actuator subsystem and light sensitive elements are arranged near the conduits of the first and/or the second mounting plate and are connected to a control unit which adjusts the position of the slideable optical elements such that signals caused in the light sensitive elements by improper alignment of optical signals are reduced to a minimum. Two or more light sensitive elements are preferably placed at regular distances on a circle concentric to a conduit. The slideable optical elements are preferably designed to provide optical signals with a main lobe and at least one side lobe which is directed toward a light sensitive element. The control unit may sense data carried by the signals received from the light sensitive elements.
The slideable optical elements may be designed to have a fan-out of at least two optical beams which are guided along different switching paths. The slideable optical elements are preferably mounted between the corresponding mounting plate and a holding plate comprising apertures for the passage of optical signals.
The first and second mounting plates may be connected to each other by spacers or side-walls.
Viewing the present invention from another aspect, there is now provided a method for switching an optical signal from a first optical fiber along one of a plurality of selectable switching paths each terminating in a corresponding one of a plurality of second optical fibers via an optical element, the optical element being moveable by a controllable actuator from a first to a second position to change the switching path of incident optical signal, characterised in that the method comprises: slideably mounting an optical element in parallel to a first mounting plate comprising a conduit; and directing the optical signals from the first optical fiber through the conduit by the optical element along the selected one of the switching paths to one of a plurality of conduits in a second mounting plate parallel to the first mounting plate, and further to the corresponding one of second optical fibers.
In a preferred embodiment of the present invention to be described shortly, optical signals are passed from a first optical fiber, directed by an optical element along a selectable switching path, to a second optical fiber. The optical element is moveable by a controllable actuator from a first to a second position in order to change the switching paths of incident optical signals. The optical elements are slideably mounted in parallel to a first mounting plate comprising conduits through which the optical signals from the first optical fiber can be directed by the optical elements to the second optical fiber via conduits in a second mounting plate aligned in parallel to the first mounting plate. Preferably, optical elements are also mounted slideable in parallel to the second mounting plate in order to direct optical signals arriving from the second optical fibers to the optical elements of the first mounting plate and further to the first optical fibers. Each of the slideable optical elements is coupled to an actuator or actuating elements capable to slide the optical elements between different positions within a plane in parallel to the corresponding mounting plate in order to establish a selectable switching path. This therefore allows switching of optical signals bi-directionally in three dimensions between optical fibers having access points on the device which are distributed in two dimensions on a first and a second mounting plate. The resulting switching paths enclosed between the two mounting plates which are installed in parallel to each other can therefore be kept at small size while switching the optical signals of numerous fibers. At the same time the deflection angles on the switching paths can be kept low which ensures fast and precise switching. At the same time beam aberration, particularly beam broadening, while passing along the switching path, is minimised.
Besides improved capabilities and functionalities, devices embodying the present invention provide advantages in the production processes. Specifically, devices embodying the present invention can be produced in parallel layers, which reduces production time and alignment efforts. The optical elements are slideable in parallel to the mounting plates without the use of panels and hinges which may be of reduced stability. The aforementioned actuators for devices embodying the present invention which move the slideable optical elements allow a precise positioning of the optical elements which then can be securely held by friction in a selected position until the switching path is changed again. As mentioned earlier, the slideable optical elements may comprise a reflective and a refractive part such as a mirror and a lens respectively so that the operational mode of the device can be changed between transmission and reflection. Also, as mentioned earlier, light sensitive elements may be arranged near the conduits of the first and/or the second mounting plate through which the optical signals are guided in order to establish and adjust the switching paths. Misalignments of the switching paths can therefore be detected and corrected by means of a control unit which adjusts and optimises the position of the optical elements by feedback.